1. Field of the Invention
This invention relates to an apparatus and method for boosting millivolt level voltages into higher voltages.
2. Background of the Invention
Voltage is a measurement of the electrical potential between two points.
It is the driving force that causes electrical current to flow. Most battery powered electrical devices use one or more series connected electrochemical cells. Such electrochemical cells, when fully charged, generally have an open circuit voltage of between 1.2 and 4.2 volts. There are some devices that, due to size, weight or cost constraints, must operate off a single cell. In such devices, a voltage boost circuit is often used to take the relatively low voltage of a single cell (which might be under 1 volt for a partially discharged cell) and increase the voltage to a higher level that is more favorable for powering electronic circuits. An example is a laser pointer that can work off of a single cell by using a voltage boost circuit that takes the low voltage from a single electrochemical cell and boosts the voltage to the 4 to 5 volts necessary to drive the laser diode. Another example is the voltage boost circuit that is common in cameras to drive a flash lamp. A single cell, or two series connected cells provide a battery voltage of 1-3 volts. This is then boosted electronically to charge a capacitor to over 200 volts. When the flash is activated, the charged capacitor is rapidly discharged through the flash lamp.
Voltage boost integrated circuits that are designed to boost relatively low input voltages are commercially available. One example is the Texas Instruments TPS61200 low voltage synchronous boost converter which is designed to operate from an input voltage as low as 0.3 volts. However, there are circumstances where it is desirable to boost a much lower voltage. An example is the voltage that is produced from a thermoelectric generation device. Such devices are composed of series connected thermoelements. The generated voltage from a thermoelectric device is proportional to the temperature across the device. So, any given thermoelectric generator will output a voltage of zero volts when there is zero temperature difference and will output a small voltage when there is a small temperature difference. As such, a voltage boost circuit that can convert a very low voltage into a more usable higher voltage, has great value in opening the door for thermoelectric generator applications that have not been previously been possible, allowing useful energy extraction from the heat energy flowing through a thermoelectric device, even when it is driven by a relatively low temperature differential.
It is common in the art to construct voltage boost designs using energy storage components, such as capacitors and inductors, as part of the voltage step-up. One or more transistor switches are used to alternatively either deliver power to, or extract power from, an energy storage component in order to boost the output voltage. The drive for these transistor switches is often obtained from an external source as in U.S. Pat. No. 4,890,210 (Myers). However, there is a need to drive these one or more transistor switches without the use of external oscillators. U.S. Pat. No. 6,597,155 B2 (Huang et al) discloses a self-oscillation boost DC/DC converter that purports to operate from voltages lower than 5 volts DC and that uses two NPN type transistors configured so that when one transistor is turned on, the second transistor is turned off. This circuit requires a minimum of two transistors and cannot initiate oscillation for input voltages of less than the base-emitter voltages of the transistor, typically on the order of 0.6 volts. U.S. Pat. No. 7,170,762 B2 (Chian et al) discloses a low voltage DC-DC converter that accepts input voltages in the range of 100 mV to 700 mV and converts this to a higher level DC voltage. The disclosed embodiments draw power from the source only during alternating half cycles and have the further disadvantage of using a capacitor that is used as part of the self-start oscillator, thereby attenuating the voltage on a transistor gate, and preventing oscillation at very low voltages. A resistor in that circuit also serves to dissipate gate energy and attenuate the gate drive voltage. Co-pending application U.S. Ser. No. 12/333,520 (Rubio et al) discloses a circuit topology for boosting very low voltages that is self-starting and utilizes a single transistor.
The present invention uses self-resonance to cause oscillation when an input voltage is present. That input voltage can be as low as 20 millivolts (0.020 volts) or less, depending upon the circuit parameters. By exploiting the unique properties of a single depletion mode junction field effect transistor (JFET), start-up occurs automatically without the requirement for signal attenuating capacitive coupling at the gate. By then using the oscillations of the depletion mode JFET to control the conduction of one or more N-channel MOSFETs, it is possible to boost the electrical current coupled from a voltage source into the primary side of a transformer, resulting in enhanced power transfer to the secondary side of a transformer. By adding an second primary winding that has a reverse winding sense from the first primary winding, and by controlling current flow through this second primary winding using one or more P-channel MOSFETs, it is possible to obtain power draw from the source during all half cycles, resulting in improved efficiency. This is a so-called push/pull configuration. By using two such voltage boost circuits in opposing polarity with respect to the input voltage, a voltage boost circuit may be constructed that accepts either polarity of input voltage and boosts it to a higher level DC output voltage of known polarity.
To summarize, the present invention has the following objects and advantages:                a) It allows the step-up voltage conversion of an ultra low input voltage into a more usable higher voltage;        b) It does not require an external input to start or to maintain oscillations;        c) it requires only a single transistor to initiate and sustain oscillations;        d) It does not require capacitive coupling into the transistor gate;        e) It allows practical power extraction from thermoelectric generation elements even when the temperature gradient across those elements is very low;        f) In some configurations, it can operate in push/pull mode, allowing full wave extraction of energy from the voltage source;        g) It produces an oscillating output that can feed into a voltage multiplier to produce different direct current voltage levels;        h) it may be configured to accept input voltages of either polarity; and        i) It is a more efficient energy converter from low voltage to higher voltage that prior art designs.        
Other objects and advantages will be apparent from the detailed drawings and description to follow.